Performance of the Conventional Divertor in TRT

Plasma Physics Reports - The paper presents the first results of SOLPS4.3 modeling of the TRT divertor within the conventional paradigm: a single-null poloidal divertor with solid metal...     

Spontaneous Break of up–down Symmetry in a Symmetric Double-Null Divertor Configuration

Plasma Physics Reports - A possibility of up–down asymmetry of the profiles of the edge plasma parameters in a symmetric double-null divertor configuration in the case of impurity seeding is...     

Spatial–Temporal Properties of Current-Convective Turbulence in Divertor Plasma under DIII-D-Like Detached Conditions

Plasma Physics Reports - Recent experimental observations of divertor plasma turbulence at the ASDEX Upgrade tokamak have demonstrated the onset of strong fluctuations of plasma radiation intensity...     

Plans for Liquid Metal Divertor in Tokamak Compass

The COMPASS tokamak (R = 0.56 m, a = 0.2 m, B_T = 1.3 T, I_p ~ 300 kA, pulse duration 0.4 s) operates in ITER-like plasma shape in H-mode with Type-I ELMs. In 2019, we plan to install into the divertor a test target based on capillary porous system filled with liquid lithium/tin. This single target will be inclined toroidally in order to be exposed to ITER-relevant surface heat flux (20 MW/m²). Based on precisely measured actual heat fluxes, our simulations predict (for 45° inclination, without accounting for the lithium vapor shielding) the surface temperature rises up to 700°C within 120 ms of the standard ELMy H-mode heat flux with ELM filaments reaching hundreds MW/m². Significant lithium vaporization is expected. The target surface will be observed by spectroscopy, fast visible and infrared cameras. The scientific program will be focused on operational issues (redeposition of the evaporated metal, ejection of droplets, if any) as well as on the effect on the plasma physics (improvement of plasma confinement, L–H power threshold, Z_eff, etc.). After 2024, a closed liquid divertor may be installed into the planned COMPASS Upgrade tokamak (R = 0.84 m, a = 0.3 m, B_T = 5 T, I_p = 2 MA, P_in = 8 MW, pulse duration ~2 s) with ITER-relevant heat fluxes loading the entire toroidal divertor.     

Concept of the divertor of a fusion neutron source based on a spherical tokamak

A concept of the divertor and the technology for organizing the edge plasma in a fusion neutron source based on a spherical tokamak (FNS-ST) are described. The experimental data on the characteristics of the peripheral plasma in modern tokamaks are extrapolated to the FNS-ST conditions with the help of semi-analytical models. The effects depending on the magnetic configuration and on the geometry and materials of the divertor and the first-wall elements are considered. Possible designs of the FNS-ST divertor and the first wall are described. Using an original model, it is shown that the maximum density of the heat flux at the divertor plates in a double-null magnetic configuration does not exceed 5?C6 MW/m², which complies with modern engineering capabilities. Methods for further improvement of the FNS-ST divertor concept are analyzed.     

Hydrogen lines in the infrared region and spectral background for the thomson scattering diagnostics of the iter divertor plasma

Calculations are made of the plasma spectral background, which is important for the Thomson scattering diagnostics in the ITER divertor. Theoretical grounds have been elaborated for computing the hydrogen spectral line shapes in the infrared spectral region for a divertor plasma in ITER. The shape of the P-7 Paschen line (transition n = 7 → n = 3) located near the laser scattering signal has been calculated for various lines of sight in the ITER divertor. Contributions from different mechanisms of broadening the P-7 line have been examined. The spectral intensities of bremsstrahlung and photorecombination continuum have been calculated. All calculations use data on the spatial distribution of temperatures and densities of all species of plasma particles computed with the SOLPS4.3 code for basic operation regimes of the ITER divertor.     

MHD stability of a plasma in an axisymmetric open divertor configuration

For a plasma with confined in a system of two simple axisymmetric mirror cells separated by a divertor cell, a radial plasma pressure profile is obtained that is stable against convective modes and drops off to zero at the separatrix. The shape of the marginally stable pressure profile depends on the geometric parameters (such as mirror ratios and the localization of the divertor cell), the ratio of the pressure in the mirrors cells to the pressure in the divertor cell, and the degree of pressure anisotropy.     

Plasma equilibrium in axisymmetric open divertor configurations

The equilibrium of a plasma with isotropic pressure in a periodic divertor configuration with a poloidal magnetic field is calculated. The issue of how the plasma equilibrium changes as the parameter β≡8πp/B ² increases is considered for a fairly representative class of pressure profiles p(ψ) (where ψ is the flux coordinate). It is shown that the plasma can be in equilibrium up to β values (in terms of the vacuum magnetic field at the divertor axis) on the order of unity.     

SOLPS4.3 Modeling of Lithium Transport and Noncoronal Radiation in the T-15 Tokamak with Lithium Emitter−Collector Scheme in Use

The lithium emitter?collector scheme has been successfully used on limiter tokamak T-11M for a long time and is therefore one of the most mature concepts of liquid lithium-assisted power and particle exhaust. In the present paper, a possible application of the emitter?collector scheme to the divertor tokamak T-15 is analyzed with the 2D transport code SOLPS4.3. Modeling indicates that placing the lithium limiter at the outer midplane of the vessel results in the widest possible spreading of lithium over the scrape-off layer, whereas lithium deposition in this case is localized primarily at the upper outer target plate. The power exhaust capability of the lithium emitter?collector scheme is also studied. It is often presumed on the basis of a simple 0D analysis that the noncoronality can bring the lithium radiation capability in line with the other low-Z impurities typically involved in the divertor power exhaust (such as carbon and nitrogen). However, detailed 2D modeling shows that in spite of the significant increase of the Li cooling rate due to the noncoronal effects, the contribution of lithium radiation to divertor power balance remains marginal, unless the lithium inventory in the edge becomes close to the deuterium one.     

Possible mechanism for the onset of vertical asymmetry of diverted plasma fluxes in a torsatron

Another mechanism is proposed to explain the onset of vertical asymmetry of diverted plasma fluxes in toroidal plasma confinement systems with a helical divertor configuration. The mechanism is governed by the possible regular displacement of plasma particles in a certain direction, e.g., due to the rigid rotation of the plasma column.     

Measurements of the Plasma Parameters in a Mirror Trap by Means of Laser-Induced Fluorescence

Results of measurements of the absolute density of ArII ions in a mirror trap with a toroidal divertor are presented. The ion density was measured by the method of laser-induced fluorescence with the use of ArII metastable states. A technique is proposed to estimate the plasma electron temperature from the ratio of the fluorescence signals obtained by comparing the intensities of different optical transitions of ArII.     

In vitro evaluation of flow divertors in an elastase-induced saccular aneurysm model in rabbit

Endovascular coiling is an acceptable treatment of intracranial aneurysms, yet long term follow-ups suggest that endovascular coiling fails to achieve complete aneurysm occlusions particularly in wide-neck and giant aneurysms. Placing of a stentlike device across the aneurysm neck may be sufficient to occlude the aneurysm by promoting intra-aneurysmal thrombosis; however, conclusive evidence of its efficacy is still lacking. In this study, we investigate in vitro the efficacy of custom designed flow divertors that will be subsequently implanted in a large cohort of animals. The aim of this study is to provide a detailed database against which in vivo results can be analyzed. Six custom designed flow divertors were fabricated and tested in vitro. The design matrix included three different porosities (75%, 70%, and 65%). For each porosity, there were two divertors with one having a nominal pore density double than that of the other. To quantify efficacy, the divertors were implanted in a compliant elastomeric model of an elastase-induced aneurysm model in rabbit and intra-aneurysmal flow changes were evaluated by particle image velocimetry (PIV). PIV results indicate a marked reduction in intra-aneurysmal flow activity after divertor implantation in the innominate artery across the aneurysm neck. The mean hydrodynamic circulation after divertor implantation was reduced to 14% or less of the mean circulation in the control and the mean intra-aneurysmal kinetic energy was reduced to 29% or less of its value in the control. The intra-aneurysmal wall shear rate in this model is low and implantation of the flow divertor did not change the wall shear rate magnitude appreciably. This in vitro experiment evaluates the characteristics of local flow phenomena such as hydrodynamic circulation, kinetic energy, wall shear rate, perforator flow, and changes of these parameters as a result of implantation of stentlike flow divertors in an elastomeric replica of elastase-induced saccular aneurysm model in rabbit. These initial findings offer a database for evaluation of in vivo implantations of such devices in the animal model and help in further development of cerebral aneurysm bypass devices.     

Parameters of a tokamak reactor operating at a fixed fusion power as functions of the aspect ratio

Such parameters of a tokamak reactor as the major radius, plasma current, reactor volume, neutron load to the first wall, heat load to the divertor plates, and bootstrap current are studied as functions of the aspect ratio, assuming that the reactor operates at a fixed power. The conclusion is drawn that a demonstrative prototype reactor should be based on a large aspect ratio tokamak.     

Determination of the electron density in the tokamak edge plasma from the time evolution of a laser-induced fluorescence signal from atomic helium

The time evolution of a laser-induced fluorescence signal from neutral helium He I in the edge and divertor plasmas in modern magnetic confinement devices is considered. Computations are performed on the basis of a nonstationary collisional-radiative model involving ten singlet and nine triplet states of helium that affect the time evolution of the fluorescence signal. A new method is proposed for determining the electron density from measurements of the time derivative of the profile of the fluorescence spectral line.     

Fast ion loss-driven H-mode transition in RF discharge plasmas of the Uragan-3M torsatron

In the l = 3 Uragan-3M (U-3M) torsatron with a helical divertor the plasma is produced and heated by RF field in the ω ≤ ω _ci range of frequencies. A two-temperature ion perpendicular energy distribution with a suprathermal tail sets in with heating. If the heating power is high enough, a spontaneous transition to an H-like confinement mode is observed. Recently, it has been supposed that the transition is connected with hotter and suprathermal ions (common name “fast ions“, FI) loss. The objective of this work is an experimental elucidation of the real link between the H-transition and FI loss. To do this, a transient regime of the RF discharge with two H-mode states is chosen, and the evolution is followed of electron density, FI content in the confinement volume, FI outflow to the divertor and edge potential. On the basis of juxtaposing of these processes, a conclusion is made that the H-mode transition in U-3M is really driven by FI loss. Possible mechanisms resulting in the transition are discussed, among them the ion orbit loss and the radial drift of helically-trapped ion orbits seem most probable.     

Use of internal current rings in long closed magnetic confinement systems

A closed magnetic confinement system is considered in the shape a corrugated torus into one or several mirror cells of which current rings are introduced that reverse the magnetic field on the axis. An internal current ring surrounded by plasma creates a magnetic configuration with an average magnetic well on the axis. The axial plasma region of such a configuration is stabilized by cusps, whereas the outer region can be stabilized by a divertor, provided that the plasma pressure gradually decreases toward the periphery. The use of internal current rings may be profitable in stellarators in which the confinement region can be divided into several regions by magnetic mirrors.     

Plasma equilibrium in a double-dipole magnetic confinement system with a separatrix

Results are presented from theoretical studies of plasma equilibrium consistent with the convective stability of ideal interchange modes in axisymmetric configurations with an outward-decreasing field that may have a separatrix limiting the plasma volume. A two-dimensional numerical code is developed to solve the Grad-Shafranov equation with a convectively stable pressure distribution at an arbitrary value of β. The problem is solved for an actual geometry of the magnetic field produced by thin current rings. Configurations of a double-dipole confinement system are calculated for the parameters measured in experiments carried out in the Magnetor device, as well as for higher β values. A configuration of a model mirror system with a divertor is also calculated. The code allows one to optimize confinement systems operating at high β values at which equilibrium still can exist.     

Laser spectroscopy measurements of the effective temperature of argon ions in the PNX-U plasma neutralizer

Laser spectroscopy measurements of the effective temperature of Ar¹⁺ ions in the PNX-U multipole trap, in which argon plasma is ionized and heated by microwaves under electron-cyclotron-resonance conditions, are performed using a narrow-band tunable dye laser. The absorption profile of the 611.5-nm line is examined. In a microwave power range of 5–50 kW, the observed behavior of the effective temperature of argon ions T_{i, eff} indicates an anomalous mechanism for ion heating. It is shown that certain information about the electron temperature can be deduced from measurements by the laser-induced fluorescence (LIF) technique. The measurements performed also serve to test the laser technique and apparatus that is presently being developed for diagnosing additives to the ITER divertor plasma by the LIF method.     

Particle confinement in axisymmetric poloidal magnetic field configurations with zeros of B: Methodological note

Collisionless particle confinement in axisymmetric configurations with magnetic field nulls is analyzed. The existence of an invariant of motion—the generalized azimuthal momentum—makes it possible to determine in which of the spatial regions separated by magnetic separatrices passing through the magnetic null lines the particle occurs after it leaves the vicinity of a magnetic null line. In particular, it is possible to formulate a sufficient condition for the particle not to escape through the separatrix from the confinement region to the external region. In the configuration under analysis, the particles can be lost from a separatrix layer with a thickness on the order of the Larmor radius because of the nonconservation of the magnetic moment μ. In this case, the variations in μ are easier to describe in a coordinate system associated with the magnetic surfaces. An analysis is made of the applicability of expressions for the single-pass change Δμ in the magnetic moment that were obtained in different magnetic field models for a confinement system with a divertor (such that there is a circular null line).     

Impurity radiation from a tokamak plasma

In tokamak operating modes, energy balance is often governed by impurity radiation. This is the case near the divertor plates, during impurity pellet injection, during controlled discharge disruptions, etc. The calculation of impurity radiation is a fairly involved task (it is sometimes the most difficult part of the general problem) because the radiation power is determined by the distribution of ions over the excited states and by the rate constants of elementary processes of radiation and absorption. The objective of this paper is to summarize in one place all the approximate formulas that would help investigators to describe radiation from the most often encountered impurities in a fairly simple way in their calculations accounting for plasma radiation, without reference to special literature. Simple approximating formulas describing ionization, recombination, and charge-exchange processes, as well as radiative losses from ions with a given charge, are presented for five impurity species: beryllium, carbon, oxygen, neon, and argon. Estimating formulas that allow one to take into account plasma opacity for resonant photons in line impurity radiation are also presented.